Ion source



Feb. 28, 1950 L; p. 9. KING 2,498,841

ION- SOURCE Filed June 1, 1945 I 3 Sheets-Sheet 1 IN V EN TOR.

wa M- L. D. P. KING Feb. 28, 1950 ION SOURCE s Sheets-Sheet 2 Filed Juhe1, 1945 AWN ' INVENTOR. L. DPez-civ'al BY Feb. 28, 1950 Filed June 1,1945 L. a. P. KING 2,498,841

ION SOURCE I 3 Sheets-Sheet 3 IN V EN TOR.

L. Percival [fizz BY y Patented Feb. 28, 1950 ION SOURCE L. D. PercivalKing, Santa Fe,-N. Mex., assignor to the United States of America asrepresented by the United States Atomic Energy Commission ApplicationJune 1, 1945, Serial No. 597,019

3 Claims.

This invention relates to a gaseous electric discharge apparatus andmore particularly to apparatus capable of producing in quantity asubstantially continuous stream of ions to be em- 'ployed in a chargedparticle accelerating device such as a cyclotron or the like. Moreparticularly, the electric discharge apparatus of the present inventionis designed primarily for readily producing a substantially continuousstream of gaseous ions under critical conditions.

In normal accelerating procedures it is found desirable and/or necessaryto accelerate as many of the desired ions as possible in order to reducethe operating time of the accelerating device with resulting savings inthe power expended. Furthermore, when inefiicient ion sources areutilized, gases are pumped through'the accelerating devices withoutproducing ions causing interference with the proper operation of thedevice in addition to the waste involved in the pumping work and in thenon-productive use of expensive raw materials.

For example, the importance of the considertions mentioned above may begauged from such nuclear reactions as are brought about by bombardmentin a cyclotron or the like by accelerated deuterium and/or tritium ions.Deuterium ions are extremely important among the many bombardingparticles used for transmutation purposes.

essary ion beam intensity at the target (1. e., the beam current asmeasured in microampere units) renders the problem prohibitive from thestandpoint of expense and equipment when ion source apparatusconstructed in keeping with the teachings and principles of the priorart are followed. Similarly, atomic tritium ions, that is ions of thehydrogen istope of mass 3, are extremely costly and rare since theistope can only be obtained in concentrations of the order of 0.025%after careful separation. Studies of the nuclear changes brought aboutby bombardment by tritium ions are important, for example, to fullyunderstand the nature of the reaction produced by the bombardment ofdeuterium with deuterium ions and the use of this latter reaction as ameans for supplying a substantial number of neutrons such as may beuseful for example in initiating and/or producing a neutron chainreaction. M Since such nuclear processes are infrequent in labonatoryoccurence, it is necessary to have higher beam currents at the targetzones and a source that will function without interruption to producesuch a beam in order to obtain a full and complete The gas, however, iscostly and the pump-v ing of large volumes in order to attain the nec-'2 understanding of the various factors involved. Illustrative of themethods previously used when the gas pressure in a cyclotron ismaintained at about 10 millimeters of mercury (as it must be to preventelectrical discharge within the device) the introduction of'heatedfilament at the central region of the device and adjacent to theentrance apertures of the electrodes or Ds of the device was found to besufiicient to give beam intensities of from one to ten microamperes.Higher intensities (e. g. of the order of one hundred microamperes)could be obtained by lengthening the filament and increasing thevertical height of the apertures. However, such a procedure required alonger magnetic gap in the cyclotron and consequently involved a greaterpower input to obtain the required field intensities. More compact andeflicient ion sources have been designed to overcome the deficiency justmentioned as well as to correct the divergent nature of a beam producedby an elongated filament. Many such sources were of the electricdischarge type and included a cathode and an anode contained within anenclosed chamber, means for the introduction of a gas into the chamber,means for establishing an are between the cathode and anode, and acapillary passage aligned with the entrance to the acceleratingapparatus through which the gas passed and was ionized. By form ing theare between an electrode and the capillary passage as is disclosed inSlepian U. S. Patent 2,285,622 or by constructing the arc discharge in apassage which communicates with the capillary outlet, it was foundpossible to increase the ion production in such electric dischargesources. However, the increased ion production was accompanied by acontinuation of the divergent nature of the beam requiring thecombination of focusing parameters or electro-static lenses with thesource. Furthermore, while such studies in electron optics and the likehave advanced, they have failed to supply a flexible method .orapparatus for producing intense positive ion beams, more particularly,an ion source that is readily adaptable to supply ions of various gases,having different characteristics, as deuterium, helium, tritium and/orthe like.

From the above, it becomes apparent that the present invention has foran object the provision of a simple compact method and means forproducing a substantial number of gaseous ions.

Another object is to provide a method of focusing an intense ion beam.

It is a further object to provide flexible focuslng means which willalso serve to effect a. preliminary separation and/or elimination ofundesirable ions from an accelerating device.

It is still further object of this invention to provide an electricdischarge type of ion source which will operate for considerable periodsof time without burning out or sufiering from other eifects engenderedby high current densities required for operation.

Other objects will appear from the following detailed description of theinvention.

The objects mentioned are attained by the present invention byestablishing a collimated beam of fast moving electrons which isdirected into a capillary opening in an otherwise sealed closure orchamber into which the gas to be ionized is introduced at a. pressure atleast in excess of the pressure maintained at the outside of the chamberand adjacent to the outward end of the capillary opening or passage. Asthe gas moves out of the chamber, by reason of the pressuredifierential, the atoms are bombarded in the capillary by the collimatedelectrons and are ionized and given an outward acceleration by anelectric field maintained outside the source. Now, by placing adeflecting surface in the path of the outwardly directed and acceleratedstream of ions and causing the stream to impinge upon that surface, ithas been discovered that if the diameter of the capillary opening of thechamber and the angle which the incident ion beam makes with t thatsurface are properly chosen, the desired ions can be directed in asubstantially sharply focused beam into the accelerating device and alarge number of undesired ions can be deflected away from the entranceof the accelerating device.

In order to further the'understanding of this invention, reference ismade to an illustrative embodiment which is only given by way of exampleand should not be deemed a limitation hereon. The principles of theinvention are applied to the construction of an ion source of theelectric discharge type for use in a cyclotron although as will be moreapparent from the detailed description hereinafter set out, a similar orequivalent structure may be used for producing ions for other chargedparticle accelerating devices or other uses. In the drawings, which aremade part of this specification Figure 1 shows in plan an electricdischarge type ion source constructed according to the principles ofthis invention.

Figure 2 is an elevational view partly in crosssection taken on the line2--2 in Figure 1 which shows the position of the filamentary electronemissive cathode used in this embodiment.

Figure 3 is another sectional view of the ion source taken on the line33 in Figure 2.

Figure 4 is a schematic plan representation showing the position of theion source in a cyclotron structure.

Figure 5 is a schematic elevation showing the vertical disposition ofthe ion source in relation to the accelerating device.

Turning now to the drawings, the ion source embodying the principles ofthis invention consists of an elongated fiat source housing 5 whichcommunicates with an enlarged hollow cylindrical portion l made integralwith a header plate 8 by soldering or in any other suitable way thatwill result in an air tight seal. The header plate 8 extends beyond thecylindrical. section l to form a circumferential flange which abutsagainst a cooperating raised portion or boss on the outer wall of thecyclotron and thereby acts to properly 4 position the source in relationto the cyclotron electrodes as will be more fully explained hereafter.

The flat portion 8 of the housing is preferably constructed in themanner indicated more clearly in Figure 2. A copper tube 9 having aninside diameter of about three-eighths of an inch and a wall thicknessof about one-thirty-second of an inch is bent in the shape of anelongated U. The straight portions of the bent tube 9 should besufficiently long to permit proper positioning of the source, theirlength depending upon the diameter of the cyclotron housing andallowance of a sufficient excess for the connection of inlet and outletwater pipes, to the portion of the tubes that extend beyond the header8. Copper strip it) which is about one-eighth of an inch thick issoldered to the tube 3 to form the bottom or" the fiat portion 6 andcopper strip H is soldered to the tube 9 to form the top. The strip i lis one-eighth of an inch in thickness for most of its length but has aheavier portion, for example, about onequarter of an inch in thickness,which is drilled and threaded to accommodate the hollow conical copperare chamber housing 82 in heat conducting relationship. Now if thestrips Ill and H are properly soldered to the tube 9 so that gas leakageis prevented a flat housing will be formed which has a conical arcchamber housing removably inserted in gas tight relationship therewith,in the upper side. It might also be noted at this point that it ispreferred to thread the conical chamber housing l2 externally as isshown in Figures 2 and 3 to cooperate with an internally threadedopening in the strip ll because upon heating of the walls of chamberhousing l2 by the action of the arc, expansion will tend to close thecontacting threads and thus tend to prevent gas leaks during operation.Furthermore, it is found desirable to provide a shoulder on the arcchamber 82 to abut against the strip ii to provide maximum heat transferacross the interface of the two parts.

Referring now to Figures 2 and 3 of the drawings wherein certainfeatures of the invention are shown in more detail, it will be noted.that the cathode i3 is a helix of a suitable refractory metal wire,preferably tungsten, and is supported within the arc chamber i i byhaving its ends secured within drilled openings in a respective one ofthe copper blocks l5 by set screws, for example. The copper blocks 55are in turn supported in spaced relationship by quartz, lviycalex orother insulators is tightly seated in recesses provided therefor inbottom strip ill. Alternatively, the supporting blocks iii may beinsulated from the strip it by sheets of mica or other heat resistantinsulating material. Hollow electrical conductors H and 22 are solderedto the blocks 55 and extends through the flat chamber 6 and areinsulated and sealed from the header plate 8 by the bushings it. Aninner squirt tube is supported coaxially and concentrically within theouter conductor li permits a liquid coolant to be circulated through thesquirt tube 19 and returned through the annular space between the squirttube i5 and the hollow conductor 3. The outward end of conductor ll issealed with squirt tube 59 by soldering for example, thus permitting thecirculating coolant to be tapped from the conductor :1 l at somesuitable place, for example, as is shown in Figure l. The provision forthe circulation of a liquid coolant to the blocks l5 permits the removalof heat from the ends of the filamentary cathode where they contact theblocks l5 and thus minimizes the deterioration of the filament.

Since a suitable heater voltage must be applied to the filamentarycathode and further since an accelerating voltage for electrons must beapplied between the filamentary cathode and the arc chamber walls as ananode for causing the electron emission of the filament to bombard thegaseous atoms within the source it is apparent that the conductors [land 22 must be insulated from each other. As has been noted they areeach insulated and sealed from the header plate 8 by the bushing l8.Glass wool or quartz tube sleeves fitted over each of the conductors lland 22 prevent a discharge between the conductors within the sourcehousing.

It will be seen from the foregoing that provision is made for acompletely water cooled ion source housing structure in which aredisposed water cooled conductors for supplying a heater voltage to afilamentary cathode disposed within such housing. It has been foundsatisfactory to connect the liquid coolant circulating systems formed bythe squirt tubes [9 and 2| and the conductors I! and 22 respectively inseries as is shown in Figure 1. In other words, a liquid coolant is ledinto the squirt tube I9 through a suitable hose connection to a sourceof such liquid coolant, and circulates to the block l5 and returnsthrough the annular space between the squirt tube and the conductor lland is led therefrom through rubber hose connection 23 to squirt tube2!. Rubber tubing 2A carries the discharged coolant from conductor 22 toany convenient discharge point. Similar provisions of supply anddischarge are made for circulating a coolant through tube 9 though notshown in the drawings.

A gas, the ions of which are desired is introduced into the sourcethrough the tube 3! which is soldered or otherwise rigidly attached toheader plate 8. A flexible hose or the like is connected to the tube byclamping or by other well known coupling devices, and extends to theoutside of the cyclotron structure. Metering devices such as flowmeters, pressure meters and the like may be used to determine the amountof gas being troduced into the source, and suitable regulating valvessuch as indicated at 32 disposed outside the cyclotron may be used tocontrol these amounts.

Returning now to a consideration of the arc chamber 14 as defined by theconical housing l2 and more particularly to the view shown in Figure 2,it will be noted that the apex of the housing I2 is drilled and threadedto receive a copper deflecting member 25 which has a suitable capillarypassage 26 drilled in'it to permit communication between the arc chamberI l and the accelerating device. A lock nut 21 is provided so that thedirection of deflection might be adjustable by adjustment of theposition of member 25 (i. e. by rotation about the vertical axis ofcapillary 26) and the heat transfer between the member 25 and housing 12kept at a maximum.

The cylindrical section 1 of the housing comprises the header plate 8, atubular section 38 of copper tubing, and a sealing transition plate 39,all soldered together in gas tight relationship. Openings are providedin the sealing plate 39 and header plate 8 to permit the legs of thetube 9 to extend therethrough. The entire housing may then be sealed bysoldering the strips l0 and II to the plate 39 and suitably sealing thetube 9 at the plate 39 and at the header plate 8. A gas passage is alsoprovided in sealing plate 39 to permit communication between the spaceswithin the cylindrical section I and within the flat portion 6 andpassage of the gas, the ions of which are desired, from the supplycontainer to the arc chamber.

In operation of a cyclotron with a source of the type described, theouter housing 28 (see Figure 4) has an opening through which the sourceis inserted to the position shown. To permit insertion of the source orits removal from the housing 28 without destroying the vacuum conditionsexisting therein, it has been found desirable to employ a substantiallycylindrical housing or air-lock 29 having a sealing plate 39 attached tothe outwardly extending end. A gate valve, not shown, is interposedbetween the air-lock 29 and the housing 2.8. When the source is readyfor use with the gate valve in closed position, the source is insertedinto the air-lock 29 which is then attached to the outer side of thevalve by a multiplicity of bolts. The air-lock may then be evacuated andthe gate valve opened. A rod which engages a threaded opening in header8 and extends through a suitable sealing bushing integral with sealingplate 30 may then be used to push the ion source into a position withinthe cyclotron structure such that the desired ions are properly directedinto the accelerating electrodes 36 and 3'! (see Figures 4 and 5). Theair-lock 2% may then be removed leaving the source in sealedrelationship with the boss on the housing 28 and the necessary liquidcoolant and gas connections made for operation.

Now using an ion source such as has been described, in a cyclotronoperation in which, for example, tritium ions are desired as thebombarding particles, it was found that highly satisfactory resultscould be obtained whereas the use of previously designed ion sourcesresulted in failure. For this disintegrating operation, which involvedthe use of a gaseous mixture containing 0.025% of tritium atoms which inturn was diluted in ordinary tank hydrogen gas, in a ratio of one partof the tritium concentrate to one thousand parts of tank gas, thecapillary opening 25 in the deflecting member 25 was about 0.140 inch indiameter and the angle the deflecting surface made with the verticalaxis of the capillary was about thirty-five degrees. The overall heightof the source at the inwardly extended end was about two inches, thebeam being deflected into the cyclotron electrode 311 on a horizontalline substantially at the center of that electrode. The cyclotron was athirty-seven inch type thus the overall length of the source housing wasof the order of nineteen inches.

The source was inserted into the cyclotron housing using the air-lockprocedure explained above, the pressure in the cyclotron beingmaintained at about 10- millimeters of mercury. The switches (not shown)controlling the current input to the filament through wires 33 wereclosed and the filament heated slowly to an electron emissivetemperature, the necessary current control being maintained by avariable autotransformer or other suitable voltage regulator. The valve32 was opened and the gas mixture flowed from the storage container (notshown) through the entrance tube 3| into the source housing andparticularly into the arc chamber is. The potential difference-impressedbetween the filament l3 and the chamber walls l2 caused an arc dischargeto appear and electron bombardment in the arc to effect ionization ofthe gaseous atoms. Now, when the magnet poles 35 of the cyclotron(between which the source is disposed when properly placed in relationto the electrodes 36 and 3?) were energized so that a magnetic field ofabout five thousand gauss existed, the electron beam was stronglycollimated in a direction coaxial with the capillary and because thefilament i3 is negative with respect to the housing 52, electronacceleration takes place. The gaseous atoms escaping through thecapillary, by reason of the difierence in pressure between the chamber il and the cyclotron electrodes are thus bombarded and ionized in thecapillary by a high intensity accelerated electron beam. The ionizedparticles impinge upon the deflecting surface where a remarkable amountof separation of undesired ions takes place. Upon energizing theelectrodes with the alternating potential which acts as the acceleratingelectric field and properly tuning the frequency a sharply focused beamwas produced at the target zone of the cyclotron. With such operatingconditions prevailing, it was determined that about 5x10 tritium ionsper second were obtained at the target zone in a beam having dimensionsof one-eighth of an inch by one-hall an inch. When the original dilutionof tritium atoms is considered, the number of charged particles thusobtained indicates an extremely high efficiency in ion source operationand a relatively high degree of separation of undesired ions.

It has been found that upon the application of a surrounding magneticfield, parallel to the axis of the capillary, the energy of theaccelerated particles can be varied to a great extent by variation inthe intensity or the magnetic field and the frequency of the electricfield and sharp focusing of the particle beam accomplished. F rthe more,the size of the capillary and the angle of the deflecting member werefound to affect the charged particle separation previously mentioned.

The apparatus disclosed has been found fully operative for the purposesstated and while a pre ferred embodiment has been described for use in acyclotron, variations will be apparent to anyone skilled in the artwhereby the source may be adapted for use in other accelerating devicesor the like. For example, in the preferred ernbodiment, the magneticfield which served to collimate the electron and emergent ion beams asabove described ls supplied by the magnets which form part of thecyclotron structure. it is clear that in another type of apparatus foracceierating charged particles or for uti sing gaseous ions in otherways it may be necessary to supply a magnetic field independent of theaccelerating structure as well as an electric field to effectivelywithdraw the ions from the source. Similarly, other changes in the shapecombination of the various parts will suggest themselves to one skilledin the art based upon the teachings of this invention. Consequently, nolimitation should be placed upon the scope hereof by reason of thedescription of a preferred embodiment or the mode of operation s 5 forthexcept such ll1hitations expressed the subjoined clai "is in which it isthe intention to claim all novelty in view of the prior art.

What is claimed is:

1. Apparatus for the production of ions comprising a frusto-conicalchamber, a capillary passage positioned axially of the frusto-cone andproviding an opening into the chamber, a metal deflecting member havinga plane surface supported outwardly of the capillary passage and at anacute angle to the axis thereof, an electron source filamentaryelectrode supported in the base of the frusto-conical chamber, means forimpressing an ionizing potential between the filament and frusto-conicalchamber Walls and means for introducing a selected gas in said chamber.

2. An ion Source for a cyclotron and the like comprising a hermetichousing including a frustum chamber, a wall closing the small end of thefrustum chamber and being provided with an elongated capillary passagehaving its axis aligned with the axis of the frustum chamber, afilamentary electrode supported in the plane of the large end of thefrustum along a median line thereof and intersecting the axis thereof,means for injecting a selected gas into said chamber, means forimpressing a negative potential gradient from the filament to thechamber walls, and a conducting member supported on the small end of thefrusto-cone and having a planar surface making an acute angle with theaxis of the capillary passage.

An improved ion source apparatus for cyclotrons and the like comprisinga first base portion, a hollow stein portion and an ion generatingirustum, said base portion comprising a cylindrical housing and meansfor supporting a first end of the stem portion axially thereof, said iongenerating frustum being supported on a cylindrical base portion, andthe second end of the stem portion being affixed diametrically to thecylindrical base portion, a filamentary electrode insulatingly supportedin the cylindrical portion and provided with electrical conductors, theelectrical conductors being supported in the stem portion and leadingout through the first base portion, means for impressing a potential onthe irustum positive with respect to the filament, a capillary passageextending through the frustum small end with the axis thereof alignedwith the axis of the frustum, a deflecting planar surface supported onthe small end or" the frustum with the plane thereof defining an acuteangle with the capillary passage axis and means for introducing aselected gaseous medium into the apparatus supported on the first baseportion.

L. D. PERCIVAL KING.

SE5 CKTED The following references are of record in the lac of thispatent:

UNITED STATES PATENTS

